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J Am Coll Cardiol, 2003; 42:792-795, doi:10.1016/S0735-1097(03)00844-1 © 2003 by the American College of Cardiology Foundation |
* Cardiac Muscle Research Laboratory, Cardiovascular Institute and Cardiology Section of the Department of Medicine, Boston University School of Medicine, Boston, Massachusetts, USA
* Reprint requests and correspondence: Dr. Carl S. Apstein, Director, Cardiac Muscle Research Laboratory, Boston University School of Medicine, 650 Albany Street (Room X-727), Boston, Massachusetts 02118, USA.
capstein{at}bu.edu
However, GIK was not beneficial in patients who presented with HF, possibly because the GIK was infused at a relatively high rate, twice as high as the Estudios Cardiologicos Latinoamerica (ECLA) study (2), and caused a volume overload. In the 4% of patients who presented with mild HF (Killip class 2), GIK treatment was neither beneficial nor harmful. In the 5% who presented with severe HF (Killip classes 3 and 4), the mortality risk was higher in the GIK group than in the control group. But the small sample sizes preclude reaching statistically significant, definite conclusions about the effects of GIK in AMI patients with HF.
The results in the patients with HF are a dilemma of interpretation. Despite the small sample size and lack of statistical significance, because of their potential clinical importance, the non-significant trends cannot be completely ignored. Some insight may be gained by comparing the infusion rates and results of the current Dutch study with other large, randomized trials of GIK in AMI patients with HF.
| The ECLA experience |
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A comparison of the Dutch and ECLA studies reveals areas of agreement and disagreement. In both trials, more than 85% of the AMI patients presented without signs of congestive heart failure (CHF). Glucose-insulin-potassium in combination with reperfusion (which was done by thrombolysis in 95% of cases in the ECLA study and solely by PTCA in the Dutch study) was highly beneficial in such non-CHF cases; the relative reductions in mortality risk conferred by GIK were 72% and 66% in the Dutch and ECLA studies, respectively.
However, the ECLA results in patients who presented with HF are in the opposite direction to those of the present Dutch study. In the ECLA study there was a non-significant trend towards a lower mortality risk in the HF and shock patients who received GIK. An important treatment difference, and a possible explanation of the different results between these two studies, was the intravenous rate of volume loading, which was twice as high in the Dutch study as in the ECLA study (3 vs. 1.5 ml/kg/h). Thus, in the Dutch study, an 80-kg patient with CHF received approximately 2 l of fluid in the first 8 h.
The potential of GIK to influence mortality risk in the Killip class 3 and 4 cases in the Dutch study was also limited by the relatively low rate of successful reperfusion in these cases. Only 50% of the Killip class 3 and 4 cases had successful reperfusion; the ECLA results suggest the GIK is beneficial only in concert with reperfusion (2). In the entire Dutch study of 940 patients, only 23 Killip class 3 and 4 patients had successful reperfusion; such a small sample precludes any definite conclusions.
| Texas Heart Institute experience |
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In an experimental model of cardiogenic shock induced by multiple coronary occlusions, GIK substantially increased short-term survival (5), consistent with the Taegtmeyer results.
Considered together, these studies suggest that GIK may be potentially beneficial in AMI and post-cardiac surgery patients with HF and/or shock, but that GIK's metabolic benefit can be outweighed by excessive volume loading. In the presence of HF or shock, a reasonable strategy might employ a more concentrated GIK solution with a lower infusion rate as was done in the ECLA and Texas Heart Institute studies. Placement of a central line and careful hemodynamic monitoring would be advisable in such cases.
| Timing and duration of GIK relative to the onset and duration of ischemia |
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The fact that GIK was effective, despite being present for only a relatively brief period before reperfusion, suggests several possibilities. The GIK may protect against "reperfusion injury" or provide important metabolic support during reperfusion (6), and/or the GIK may be able to reverse some of the ischemic injury that has occurred before its being administered. Also, there is clearly a potential for increasing the benefits of GIK by its earlier administration (e.g., when the patient is first seen by ambulance personnel); this could increase the fraction of ischemic pre-reperfusion time during which GIK could provide metabolic protection. These issues require further research.
| Mechanisms of mortality reduction by GIK |
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| GIK's metabolic actions against ischemic-reperfusion injury and necrosis |
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Shifting myocardial oxidative metabolism from FFA to glucose oxidation makes the myocardium more "oxygen efficient"; approximately 11% more adenosine triphosphate (ATP) is synthesized per mole of oxygen when glucose is oxidized rather than FFA, and left ventricular (LV) function relative to oxygen consumption is improved when glucose, rather than FFA, is the major oxidative substrate (10,11). Despite a coronary occlusion, substantial oxidative ATP synthesis occurs in an acute infarct region because a significant degree of residual perfusion is almost always present (12); thus GIK has the potential to improve oxidative metabolic efficiency in the ischemic and infarcting regions.
The effect of GIK to increase glycolytic ATP synthesis is also important. During low-flow ischemia, with perfusion levels comparable to those in the infarct region in patients with AMI, provision of high glucose and insulin caused a small but significant increase in ischemic glycolytic ATP production, with consequent attenuation of both the ischemia-induced decreases in ATP and phosphocreatine and the reciprocal increases in inorganic phosphate (Pi) and adenosine diphosphate (ADP) levels (1315). The resulting combination of a higher [ATP] and lower [Pi] and [ADP] resulted in a higher calculated free energy yield from ATP hydrolysis for all cellular adenosine triphosphatase reactions.
The intracellular location of glycolytic enzymes may provide glycolytic ATP with particular value in the maintenance of critical membrane functions such as calcium and sodium homeostasis (1620). A high glucose substrate level also increases myocyte resistance to the toxic effects of the increase in cell calcium levels that occurs during hypoxia (21).
Insulin alone exerts beneficial metabolic and functional effects during ischemia (22,23). Also, insulin given during early reperfusion reduced infarct size, possibly by reducing reperfusion apoptosis (24). The net effect of these GIK actions is the reduction of cellular injury from ischemia and reperfusion, and improved ischemic and post-ischemic systolic and diastolic function (13).
| Metabolic support for the acutely loaded non-infarct region |
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| Renewed interest in myocardial metabolism, lessons learned, and future questions |
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However, the results from the Dutch and ECLA studies should not be considered conclusive; before GIK is added to the therapeutic canon, larger trials are needed. Both trials were relatively small, and in each study a statistically significant reduction in mortality occurred only in a subgroup, not in the total population studied. In the ECLA study, GIK reduced AMI mortality significantly in the subgroup that received concomitant reperfusion treatment. In the Dutch study, the statistically significant mortality reduction occurred in the non-CHF subgroup. Even though these subgroups were prospectively defined and represented the majority of patients in each study, a conclusion based on a subgroup result is not as convincing as a result from the entire study population.
Whether GIK is beneficial in all patients with AMI is a crucial issue to resolve. Approximately 1.1 million MIs occur each year in the U.S. The Dutch results suggest that approximately one million present initially without CHF and that GIK has the potential to reduce their absolute mortality risk by 3%, saving the lives of approximately 30,000 such patients each year.
Whether GIK is beneficial in AMI patients with CHF and/or shock is an equally crucial issue to resolve. Although such patients comprise a relatively small percentage of the total AMI population, they have the highest mortality risk, but are also the least tolerant of a large volume infusion. Extrapolation from the Dutch study suggests that approximately 99,000 AMIs of Killip class 2 or higher occur annually in the U.S., with a mortality risk of 26.5%, resulting in 26,235 deaths per year despite rapidly available PTCA. A trial of a relatively concentrated GIK solution with low infusion rates, such as was used by Taegtmeyer et al. (4), would seem appropriate in such cases. Because patients with CHF and shock comprise a relatively small percentage of the total AMI population, a multicenter trial is probably required to resolve this issue.
Is GIK treatment for AMI beneficial in the general community outside of specialized centers? Would its benefits be increased by starting treatment as early as possible after the onset of symptoms and giving it during transport to a hospital site of revascularization? All these issues need to be resolved. The stakes are high; although such research is costly, treatment with GIK is cheap and the potential benefits are large.
| Footnotes |
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| References |
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